Whew, I'm sorry to hear about the tough situation

When your daughter asks hard questions like these, how do you usually respond?

I would recommend that you recognize how difficult this feels for your daughter, but explain (simply) that you & her daddy have some real differences that need to be worked on. Then, change the subject to her and how much you love her. If and when she tries to turn the tables back onto the marriage, stay gentle but firm on it's an issue that she will always have your love and support but this is a decision that you're firm with.

Is your ex still using? If so, perhaps the visitations need to be changed. While he may be controlled enough to not do drugs around your daughter, she may still be in some danger with him. I'd think about that issue very thoroughly....you've got to keep her safe.